A sapphire single crystal is a material solidified in a direction with a crystal structure of a HCP (Hexagonal Close Packed) structure hexagonal system in the course of solidification after melting alumina (Al2O3) that is a compound of a combined form of aluminum (Al) and oxygen (O), at a predetermined temperature.
The sapphire single crystal is a material with a hardness next to diamond, has wear resistance and corrosion resistance about 10 times higher than quartz, and has excellent insulating and light transmission properties, which is widely used in the field of advanced materials such as IT, industrial, or military purpose, or substrates for LED, as well as for synthetic gemstones, and glass for watches. In particular, the sapphire single crystal has the spotlight as a material for a touch screen of IT equipment, and has been used as a material for windows for military infrared detection missiles, fighter aircraft, or explorer equipment.
In order to use a sapphire single crystal as windows of a precision instrument, a synthetic sapphire single crystal ingot should go through cutting, grinding and polishing processes to the shape and size of a final product. Such cutting, grinding and polishing processes, diamond abrasive whose hardness is higher than the hardness of a typical sapphire single crystal is used.
First, a cutting process is a step of cutting a synthesized sapphire single crystal ingot into a basic shape and size of a product, in which micro-cracks may occur in the thickness direction from the surface of the product due to friction between the sapphire single crystal and the abrasive during cutting. In addition, a processing stress generated during the cutting process may remain in the interior of the sapphire single crystal and thus the product may have a rough surface.
Then, the grinding process is a step of grinding a roughened surface of the sapphire single crystal more finely after cutting, in which the micro-cracks generated in the thickness direction from the surface of the product during cutting are mostly removed, but the micro-cracks generated during cutting may not be only completely removed, but also other micro-cracks that are smaller than the micro-cracks generated during cutting and a processing stress may remain in the interior of the sapphire single crystal during grinding.
Finally, the polishing process is a step of polishing the surface of the product for a light-transmitting window, in which the micro-cracks that could not be completely removed after grinding are removed in the polishing process, but the micro-cracks and the processing stress may still remain in the surface of the product even in the polishing process.
As described above, in order to use a sapphire single crystal as a window, etc., the above-described processes should be undergone, and thus the intensity of products for the sapphire single crystal windows are significantly reduced due to the micro-cracks and residual stress generated in the processes. In particular, the micro-cracks act as a starting point of fracture when a force is applied on the outside of the window and thus act as the most likely cause to lower the strength of the entire sapphire products.
A residual internal stress also acts as a cause to lower the overall strength of the product. The sapphire single crystal window whose intensity is reduced may be broken more easily when the sapphire single crystal window is used for IT equipment or military use, and may be followed by restrictions at the time of using the sapphire single crystal window as a sensor protection window or a window purpose.
As disclosed in Korean Patent No. 10-0578162 (May 2, 2006), a conventional heat treatment method for a silicon single crystal wafer is configured to include cutting a silicon single crystal ingot thinly to obtain a wafer; and annealing the obtained wafer at temperature of 1200° C. or higher for one second or more by using a rapid heating/rapid cooling apparatus, in which the silicon single crystal ingot is manufactured by increasing a growth speed to 0.6 mm/min or more, an oxygen concentration is below 16 ppma or less, and a COP whose size is 60 nm to 130 nm is present at a high density.
The conventional heat treatment method of the silicon single crystal wafer has the following defects. The silicon single crystal wafer is heated at a constant rate of temperature rise when performing the heat treatment. Accordingly, when the rate of temperature rise is large, the wafer may run a risk of damage such as a crack under the influence of heat, and when the rate of temperature rise is small, the time of temperature rise gets longer, to thereby cause productivity to drop.